In the aviation industry a requirement exists that certain control systems be reliably redundant. For components (and systems) required for safe takeoff and landing, reliably redundant means that failures of redundant components may not be latent. Failures must be patent, or detectable. Conversely, if a failure of one of the redundant components is latent, then the system fails to be reliably redundant. It will be understood by those skilled in the art that the requirement for reliable redundancy does not apply to structural components, and more particularly to load paths.
On most commercial aircraft, the captain and first officer control the control surfaces via two control sticks and two sets of rudder pedals. To control the rudder, in particular, the crew uses the two sets of rudder pedals. Both sets of rudder pedals are connected to the same single load path cable, which communicates the pilot inputs, or commands, to the aft cable quadrant and then input torque tube located at the aft end of the aircraft. The input torque tube, in turn, communicates with the feel and centering unit and trim actuator, a main power control unit and a standby power control unit. The main and standby power control units, which are directly connected to the rudder, utilize hydraulic pressure to move the rudder control surface against the air loads. The amount of hydraulic force used to move the surface via this hydro-mechanical arrangement is proportional to the air load acting on the rudder control surface.
Many commercial aircraft employ three full time, fully redundant, flight control systems so that, in the event of any combination of two simultaneous failures, the vehicle can be landed safely. Each redundant system, of course, adds weight and complexity to the vehicle. Moreover, because of the duplication of equipment, more failures are likely which tends to cause maintenance and unprofitable down time of the aircraft. Accordingly, it is desirable to provide a simple, lightweight flight control system. Yet it remains necessary to provide reliable redundancy.